Hitherto, as an ultra-low-temperature device for refrigerating an object to be refrigerated, such as a superconducting magnet or a liquid helium container that contains the superconducting magnet, an ultra-low-temperature device that uses a refrigeration device including a cold head has been known. In this ultra-low-temperature device, how efficiently the cold head is coupled to an object to be refrigerated (that is, bow small the thermal resistance can be made) and whether or not the cold head can be made separable from the object to be refrigerated are important subjects.
Patent Literature 1 discloses a device that uses a low-boiling gas, such as nitrogen, for coupling objects to be refrigerated and the cold head to each other. As shown in FIG. 6, this device includes a liquid helium container 100, a vacuum container 102 that contains the liquid helium container, a refrigeration device 106 that includes a cold head 104, a sleeve 108 that is formed between the liquid helium container 100 and the vacuum container 102 and into which the cold head 104 can be inserted from the outside of the vacuum container 102, a closing plate 110 that is mounted on a lower portion of the sleeve 108 so as to close the lower portion of the sleeve 108, a gas introducing pipe 112 for supplying heating gas, such as nitrogen gas, to a space directly above the closing plate 110, and heat transfer fins 114 that are secured to a lower surface of the closing plate 110. The closing plate 110 and the fins 114 are objects to be refrigerated by the refrigeration device 106. The refrigeration of the closing plate 110 and the fins 114 causes the temperature in the liquid helium container 100 to be maintained at an ultra-low temperature that is less than or equal to the boiling point of helium.
The cold head 104 includes a first refrigeration stage 104a at an intermediate portion of the cold head 104 and a second refrigeration stage 104b at a lower end portion of the cold head 104. The second refrigeration stage 104b and the liquid helium container 100, which is an object to be refrigerated, are coupled to each other so as to allow heat conduction as follows. First, liquid nitrogen is accumulated in a bottom portion of the sleeve 108. On the other hand, by immersing the second refrigeration stage 104b in the liquid nitrogen, the cold head 104 is inserted into the sleeve 108 at a location where a gap having a predetermined size is formed between a lower surface of the second refrigeration stage 104b and an upper surface of the closing plate 110. When, in this state, the cold head 104 is started, the second refrigeration stage 104b refrigerates the liquid nitrogen and solidifies it. This causes a thermal joint 116 formed of the solidified nitrogen to be formed. The thermal joint 116 has a high thermal conductivity, and efficiently transfers cold of the cold head 104 to the closing plate 110.
On the other hand, when, in order to, for example, maintain the refrigeration device 106, the cold head 104 is removed from the sleeve 108, the operation of the refrigeration device 106 is stopped, more desirably, heating gas (such as nitrogen gas) is introduced into the sleeve 108 via the gas introducing pipe 112. By this, the nitrogen of which the thermal joint 116 is formed is evaporated, as a result of which the thermal joint 116 disappears. This makes it possible to remove the cold head 104 from the inside of the sleeve 108.
In the device that is described in Patent Literature 1, it is difficult to prevent infiltration of heat into the objects to be refrigerated when removing the cold head 104. More specifically, in order to remove the cold head 104 from the sleeve 108, the temperature in the sleeve 108 needs to be increased to a temperature that is greater than or equal to the boiling point of nitrogen. Further, after removing the cold head 104, the inside of the sleeve 108 is open to the air. At this time, a large amount of heat infiltrates the inside of the liquid helium container 100 from the sleeve 108 via the closing plate 110.